The curing of many types of polymeric materials by means of the application of heat to activate a curing agent, or simply to incite chemical activity effecting the cure such as a cross-linking, often requires a commensurate application of high ambient pressure to offset or overcome the effects of the high curing temperatures. Unrestrained by either the confines of an enclosing mold or high ambient pressures, many common polymers are susceptible to softening and deformation at curing temperatures prior to completion of the thermosetting cure, and the high curing temperatures cause the generation or evolution of volatile gases within and about the body of the polymeric material undergoing curing. The occurrence and/or expansion of such gases in the softened and deformable polymeric material forms cells or voids within the body, resulting in a porous and low density mass of irregular consistency.
Heat curing operations for many polymeric compositions are therefore often performed within a closed chamber or steam autoclave wherein the hot confined steam, or other heated gas under high pressure provides both the high temperatures and the restraining high ambient pressure about the body undergoing curing.
"Pressureless" curing systems carried out at approximately ambient atmospheric pressure conditions are of course employed in the manufacture of many types of products. However, as indicated above, and also pointed out in page 69 of an article entitled "The Continuous Vulcanization of Solid And Cellular Profiles" by C. H. Gregory, appearing in the Rubber Journal of September 1966, the materials cured at ambient atmospheric pressure conditions generally develop considerable porosity.
Porosity, or the presence of voids or cells in a composition comprises a definite shortcoming in some types of products, and an adverse condition which cannot be tolerated in others. For instance, voids or gas cells are especially detrimental in electrical insulations, and many recent U.S. patents are addressed to the problem of porosity in electrical insulations, viz.: U.S. Pat. Nos. 3,522,183; 3,527,874; 3,629,110; and 3,666,876. The propensity of many polymeric materials, and particularly polyolefins such as polyethylene, to develop pores during heat curing, has required a practical matter the manufacturers of insulated electrical conductors to cure their polymeric insulated products under high pressure with the attendant high costs, although pressureless curing systems are effective in some other industries or products wherein the presence of pores is not a significant factor.